Astro-2: History of the Universe Lecture 2; April 2 2009 Previously.. On Astro-2 The goal of cosmology is to understand how the universe formed and evolved. How can we build a theory of the universe valid at all times in every place?

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Previously.. On Astro-2 • The goal of cosmology is to understand how the universe formed and evolved. • How can we build a theory of the universe valid at all times in every place? • We have to assume that the view of the universe from Earth now is as good as from any other place and time.

How big? The birth of Modern Cosmology • What is this? • The Milky Way! • In the past people called other galaxies Nebulae. • Nebula=cloud in Latin • In modern times Nebulae (galaxies) were studied and catalogued by Messier (M catalog) and Herschel in the XVIII and XIX century

How big? The birth of Modern Cosmology • What are “nebulae”? • Are they part of our own galaxy, the Milky Way? • Are they galaxies themselves? • Is the Milky Way all there is, or is the Universe much bigger and our galaxy is only one of the many? The closest big galaxy: Andromeda (M31) Visible with the naked eye!

How big? The birth of Modern Cosmology • We know how big is the Milky Way • 50 kpc = 4,000 billion times around the globe (universe chapter 23) • How big are the “nebulae”? • Do they fit in the Milky Way?

How big? “Island universes”? • How big are “nebulae”? • This was the subject of a heated debate in the early 1920s: • Harlow Shapley (galactic=small) • Heber D. Curtis (extragalactic=big) • By looking at them, we do not know if they are small objects nearby or big objects very far. • How do we figure this out? How big is M31?

How big? 1923 comes Hubble… • In 1923 Edwin Hubble finds the solution measuring the distance to M31. • Discovers Cepheids in M31(what are Cepheids? Universe chapter 21) • Cepheids are “standard candles”. By measuring their period, we know the intrinsic brightness. From that and observed brightness we infer distance. How big is M31?

A Cepheid in IC4182 has a period of 42 days and an apparent magnitude of m=22.0 in the V band From the period luminosity relation we know that the absolute luminosity M=-6.5 in the V band The relation between distance (in parsec) apparent and absolute magnitude is: m=M+5 log (d/pc) -5 Hence d=100.2(m-M+5)pc, i.e. 5 106 pc Cepheid distance. Example:

How big? Hubble discovers the “realm of the nebulae” • Using the Cepheid distance Hubble concludes that M31 is 750 kpc away (15 times the size of the Milky Way) • Thus, the size of M31 is 70 kpc, larger than our own Milky Way. • The same is true for billions of galaxies that populate the universe! Our Milky Way is just and “average Joe” galaxy • Overnight people realized that the universe was thousands of times bigger than they thought How big is M31? 70 kpc

How big? Answers • The universe is much bigger than the Milky Way • It contains billions of galaxies, each one tens of kpc in size. • The size of the visible universe is of order Giga (Giga=billion) pc, i.e. millions of times that of the Milky Way • It could be infinite…

What kind of stuff? The Hubble “tuning fork” diagram • Hubble classified the variety of galaxies according to their “morphology”, i.e. their appearance. • Most galaxies belong to one of these four main types: • Ellipticals • Lenticulars • Spirals. Barred and non Barred • Irregulars

What kind of stuff: Elliptical galaxies • Elliptical galaxies appear elliptical in the sky. • Sub-Classified based on the apparent (what does this mean?) elongation. • If a and b are the major and minor axis, then the galaxy is classifed as En with n = (1-b/a)*10 M87 E0 b/a=? N3377 E6 b/a=?

What kind of stuff? After shape, color, or spectrum • When you want to describe something you generally say the shape and then the color • The same things with galaxies: • First morphology, then color • A precise measurement of color is a “spectrum” • A spectrum contains lots of physical information

What kind of stuff? Spiral galaxies • Spirals are characterized by spiral arms. Sub-classified based on the relative size of the “bulge” and the “disk” • Sa have big bulges.. Sd have no bulge Sd Sa Sb Sc

What kind of stuff? Barred spiral galaxies • Barred galaxies are similar to spirals but with a boxy central feature called bar. • Bars are found in ¾ of spirals and are thought to arise from instabilities • It is unclear exactly why not all spiral galaxies are barred…

What kind of stuff? Spiral galaxies have young stars • Emission lines arise from gas “ionized” by very energetic radiation • Such high energy radiation is NOT produced by cold old stars, implying that very young stars (10 million years old) are present. • They also contain vast amounts of gas and dust Sc Galaxy

What kind of stuff? Answers • Galaxies do not take any possible form or shape. • Most galaxies belong to one of these types: • Elliptical • Lenticular • Spiral • Irregular • Ellipticals and lenticulars have stars older than the sun • Spirals and irregulars have stars younger than the sun

How heavy are spirals? • We can use rotation to measure a galaxy’s mass (“weight”) • In Equilibrium, gravity provides just the right amount of centripetal acceleration: • GM/R2=V2/R • We can use the rotation speed to infer the mass of the galaxy: • M(<R)=V2R/G

How far? Summary • Measuring distances is essential to learn how big is the universe and how much stuff there is in it • To measure distances of far away objects, more than 100kpc or so, astronomers use “standard candles” such as Supernovae • Using Supernovae or other standard candles, astronomers have been able to find a general method to measure distances. • Next, on astro-2: In the process they also discovered that the universe is expanding.